During and after the drilling of a borehole in a geological formation, certain types of downhole testing are performed in a blind fashion where downhole tools and sensors are deployed in the borehole at the end of a tubing string for several days or weeks, after which time they are retrieved to the surface. During the downhole testing operations, sensors may record measurements that will be used for interpretation. Only after retrieval to the surface does the operator ascertain whether the data obtained downhole are sufficient for the operator's purposes. In addition, while the operator may attempt to operate and control some of the downhole testing tools such as tester valves, circulating valves, packers, samplers, and perforating charges, from the surface, the operator typically cannot obtain direct feedback from the downhole tools.
While two-way communication between the surface and the downhole tools may be beneficial, such communication may be difficult to provide using a cable, as locating the cable inside of the tubing string limits flow diameter, and complex structures may be required to pass the cable from inside to the outside of the tubing. In addition, space outside the tubing is limited and cable can easily be damaged. As a result, wireless telemetry systems have value.
There are three major methods of wireless data transfer between downhole and uphole equipment: mud pulse telemetry, electromagnetic telemetry, and acoustic telemetry. Mud pulse telemetry is commonly used during drilling operations where there is mud flow in the borehole. However, whenever mud is not flowing, mud pulse telemetry cannot be utilized. In addition, mud pulse telemetry data rates are slow. Electromagnetic telemetry does not require mud flow, but does require a large amount of power. Moreover, electromagnetic telemetry is subject to noise and its success is very dependent on the formation in which the borehole is located. Acoustic telemetry is likewise not dependent on mud flow but is subject to noise, attenuation, and signal distortion due to reflections at pipe connections. Data throughput is also very limited (slow).
In order to overcome some of the issues with acoustic telemetry, and as seen in prior art FIG. 1 and FIG. 2, a linear network of acoustic modems 10 has been used to transfer data from downhole equipment 20 to the surface 30 and to transmit control information downhole. A typical bidirectional communication system may include several modems, e.g., 10a, 10b . . . , that are spaced apart along a pipe 50 and engaging (e.g., clamped to) the pipe. The modems typically include receiver and transmitter electronics 60 which are acoustically coupled to the pipes 50 by a piezostack 65, microprocessors 70, and a battery power source 80 for powering the modem as disclosed in U.S. Pat. No. 8,605,548 to Froelich, and U.S. Pat. No. 9,062,535 to Merino et al., both of which are hereby incorporated by reference herein in their entireties. While these disclosures provide important advances in overcoming noise, attenuation, and signal distortion issues, data throughput is still considered very limited in the prior art acoustic telemetry systems. In addition, because the prior art acoustic telemetry systems rely on battery power, typical systems are functional for only a couple of weeks to a month. While this signaling system life-span may be sufficient for certain testing environments, it is not sufficient for enabling signaling to continue during production.